Project 1: Integrins are a family of ubiquitous transmembrane heterodimers that mediate fundamental processes requiring cell-matrix and cell-cell interactions and reside on cell surfaces in an equilibrium between resting inactive molecules and active ligand-binding molecules. Like other proteins, integrins are soft amino acid polymers that continuously sample ensembles of conformational states. Conversion from their resting to active states occurs when allosteric modulators such the divalent cation Mn[2+] or the cytoskeletal protein talin shift the distribution of conformations from one pre-existing population to another. The resting integrin conformation is enforced by non-convalent interactions involving their membraneproximal cytoplasmic, transmembrane, and juxtamembrane extracellular domains; these interactions are disrupted when an integrin shifts to its active conformation. This project is focused on the platelet integrin allbB3 , a receptor for macromolecular ligands such as fibrinogen and von Willebr and factor (VWF) following platelet stimulation. Binding of these ligands to allbp3 is responsible for platelet aggregation and is a critical step in the formation of hemostatic platelet plugs and pathologic arterial thrombi. The overall goals of the project are gain a thermodynamic understanding of allbp3 regulation and to use this understanding to develop novel allosteric modulators to attenuate allbp3 function. In Aim 1, we will generate a thermodynamic model for allbp3 activation, testing the hypothesis that topographically-distinct interactions between allb and B3 differentially regulate allbp3 activation. The relative contribution of the membrane-proximal cytoplasmic, transmembrane, and juxtamembrane extracellular domains of allb or B3 to maintaining allbB3 in its resting conformation will be quantitated using a novel AraC-based bacterial transcriptional reporter system, as well as optical tweezers-based force spectroscopy. The data will be used to derive a thermodynamic model of allbB3 activation. Chemical libraries and molecular databases will then be screened for potential allbB3 inhibitors that bind to the relevant extracellular hot spot regions of either allb or B3. In Aim 2, we will use soluble anti-transmembrane domain peptides to modify the activation state of individual allbB3 molecules in situ. By destabilizing the allbB3 heterodimer, we found that an anti-allb transmembrane domain peptide causes ligand binding-independent transactivation of B3-bound c-Src, implying that allbB3 activation alone is sufficient to cause rapid allbB3 oligomerization. Conversely, by stabilizing the allbB3 heterodimer, transmembrane-domain targeted peptides would act as novel allbB3 antagonists. Thus, we propose using computational methods to design peptides that stabilize the allbB3 TM domain heterodimer, preventing allbB3 activation and serving as lead compounds for the development of novel antithrombotic agents.